In recent years, the popularity of mobile communications has increased immensely and is expected to grow in the near future to the point where existing systems will be unable to support the demand for such communications. A major problem facing the future of mobile communications systems is the scarcity of available bandwidth in a wireless network for the mobile user's transmission to a fixed network.
Current mobile communications systems employ the concept of "Cells." A cell is a geographical area which is assigned to a corresponding base station which is in turn wired into the fixed network. This fixed network is typically a "mesh network," and is comprised of numerous switches connected together by communication links. The mesh network is set up so that a communications route may be traced from any one switch to any other in the network through at least one, and more often, many combinations of links and switches. Some of the switches in the network, in addition to being connected to other switches, are also connected via communication links to one or more of the base stations and/or fixed termination points such as a home telephone.
When a mobile user places a call on his/her wireless terminal, the call is transmitted through the wireless network comprising radio channels, to a base station assigned to the user's cell. From the base station, the call is carried by the mesh network to the user's intended destination. When the mobile user moves from one cell to another, a call hand-off, or hand-over, between base stations takes place.
Due to the limited bandwidth available for the wireless transmissions of mobile users' calls, each cell can handle only a limited number of calls. Overload conditions occur when the communication needs of wireless terminals populating a small area exceed the total capacity of all access points within their reach. This situation is referred to as a "radio congestion state." Such a state may be encountered during a hand-off, resulting in connection dropping, long delays, and/or packet losses.
Attempts have been made to reduce the likelihood of occurrences of the radio congestion state. One such attempt requires a base station having spare capacities to accommodate some of the mobile users in a neighboring cell experiencing a congestion state. Two prior art techniques using this approach include a power control technique described in: J. Zander, "Performance of Optimum Transmitter Power Control in Cellular Radio Systems," IEEE Transactions on Vehicular Technology, vol. 41, no. 1, Feb. 1992; and a dynamic or hybrid channel allocation technique described in: S. Tekinary et al., "Handover and Channel Assignment in Mobile Cellular Networks," IEEE Communications Magazine, vol. 29, no. 11, Nov. 1991. The power control technique requires the transmission power of a base station to increase so as to effect communications with additional users in the neighboring, congested cell. The increase in the transmission power de facto causes the cell originally assigned to the base station to grow to include the neighboring cell. On the other hand, the dynamic or hybrid channel allocation technique requires that transmission frequencies or channels be rearranged so that mobile users in the congested cell can take advantage of spare capacities not utilized in other cells.
Another attempt to reduce the likelihood of the congestion state occurrence calls for application of a layer architecture for the cellular network. Such an architecture is described in: C. Iet al., "A Microcell/Macrocell Cellular Architecture for Low- and High-Mobility Wireless Users," IEEE Journal of Selected Areas in Communications, vol. 11, no. 6, Aug. 1993. In accordance with the layer architecture, an overlay cell is defined to cover multiple cells. When a cell experiences a radio congestion state, a base station assigned to the overlay cell (as opposed to the base station of the congested cell) is used to communicate with the excessive mobile users in the congested cell.
Nonetheless, the above approaches do not impose any control on the admission of new calls. As the number of new calls entering the cellular network increases, the rate of radio congestion state occurrences in such a network becomes unacceptable. Nevertheless, in prior art two connection control techniques have been formulated based on a differentiation of new calls from hand-off calls. In accordance with the technique described in: R. Guerin, "Queuing-Blocking System with Two Arrival Streams and Guard Channels," IEEE Transactions on Communications, vol. 36, no. 2, Feb. 1988, a predetermined number of guard channels at each base station are reserved for hand-off calls. Under no circumstances are these guard channels used for new calls. On the other hand, in accordance with the technique described in: Oh et al., "Prioritized Channel Assignment in a Cellular Radio Network," IEEE Transactions on Communications, vol. 40, no. 7, Jul. 1992, hand-off calls are afforded a higher priority of service by the base station over new calls. That is, given a limited number of available channels, new calls are blocked in favor of competing hand-off calls.